The present invention relates generally to reciprocating pneumatic saws and more particularly to a mechanism for preventing relative rotation between a piston unit and a body of the saw.
Reciprocating pneumatic saws are used to cut materials such as wood and metal. These saws have a body sized to be held in a user's hand and a throttle which the user actuates to control blade movement. A piston unit positioned in a cylinder inside the body drives the blade back and forth in response to air entering opposite ends of the cylinder when the user actuates the throttle. Because the cylinder and piston unit are cylindrical, the piston unit is free to turn in the cylinder, allowing the blade to rotate relative to the body of the saw.
In the past, various mechanisms have been used to prevent relative rotation between the blade and the body.
FIG. 5 shows one such conventional mechanism. This mechanism has a pair of ball bearings (designated by BB in FIG. 5) which are held in recesses formed in a blade holder H portion of the piston unit. The bearings BB engage a plate P attached to the body of the saw as the blade reciprocates to prevent the blade from rotating relative to the body. However, over time the mechanism wears and permits the blade to rotate relative to the body. Further, debris can lodge between the bearings BB and the blade holder H causing the mechanism to jam and preventing smooth movement of the blade. In addition, the mechanism requires periodic lubrication to ensure smooth movement of the blade, but the bearings are not freely accessible for lubrication. Moreover, due to the number of pieces which must be assembled and the possibility of dropping and losing the bearings, the mechanism is difficult and costly to assemble.